Arc-light carbon.



H. AYRTON.

ARG LIGHT CARBON. APPLICATION FILED MAR. 7, 1914.

1,1 1 5,480,` Patented Nov. 3, 1914.

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' f? e@ W/ //%/////////Z/// MJm/ l HERTHA AYRTON, OF HYDE PARK, LONDON,ENGLAND.

ARC-LIGHT CARBCN.

Specification of Letters Patent.

Patented Nov. 3, 1914.

Application led March 7, 1914. Serial No. 823,123.

To all whom, it may concern Be it known that I, HERTHA AYRTON, subjectof the King of Great Britain, residing at 41 Norfolk Square, Hyde Park,in the county of London, England, have invented certain new and usefulImprovements in Arc-Light Carbons, of which the following is aspecification.

This invention relates to negative carbons for arc lights.

It has not yet been possible to produce a negative carbon which may beof any required thickness and which will maintain a perfectly steady arcwith a suitable current and positive carbon under the varying conditionswhich exist in different lamp-houses or inclosures employed for arclighting.

In the drawings accompanying this speciication, Figures l and 2 arelongitudinal sections of electric light carbons which have beenheretofore used, and Fig. 3 is a longitudinal section of an electriclight carbon made in accordance with my present invention.

It is the object of this invention to provide a negative carbon whichwill fulfil these Conditions or which can be readily manufactured tosuit any particular conditions.

It is well known that to secure a steady silent arc, giving constantillumination, it is necessary that the end surface of the burning tip ofthe negative carbon shall be so small that the arc cannot wander abouton it. To overcome the various difliculties encountered with negativecarbons it has hitherto been proposed to employ very slender carbons andto coat them with copper to increase the conductivity of the carbon andthereby prevent its becoming unduly heated by reason of its reduction insize. I have also previously proposed for the purpose of obtaining aslender carbon to employ a carbon consisting of a soft core surroundedby a shell of hard carbon covered with thin copper which by increasingthe conductivity of the rod allowed of a much slenderer carbon with thesame current than could otherwise be employed. Carbons so made give verygood results for searchlights, where the inside of the lamp-house may bekept very hot, because the arc is always at a fair distance from themirror of the lens. I found that under these conditions the carbonsburnt away somewhat in the form indicated in Fig. 1 of the accompanyingdrawings, from which it will be seen that the soft core a burns awaywith a central depression in such a manner as to leave only a small ringA of hard carbon for the arc to rest upon. If, however, a carbon made asabove described is used in a lamp-house which has to be keptcomparatively cool, to avoid the cracking of the lens, I nd that theburning away of the soft corecannot always be depended upon, and attimes it may only burn away at about the same rate as the outer shell ofhard carbon b with the result that the tip of the negative carbon has nodepression in it, and presents a blunt point entailing a large endsurface as indicated in the dotted lines B Fig. l, which form is veryundesirable and produces an unsteady and noisy arc. It will therefore beseen that in the case of ordinary carbons it has been proposed for thepurpose of obtaining a slender carbon to employ in one case asubstantial thickness of copper covering, and in another case to' employa thin copper covering upon a hard shell inclosing a soft core. Furtherit has also been proposed to employ a thin copper covering to preventthe disintegration of carbons made of luminiferous material which burnin a different way from ordinary carbons, but none of these pro posalshave been entirely successful in accomplish the object of thisinvention. It has also been proposed to make negative carbons wit-hout acopper covering and in which the core a, Fig. 2, was hard, or lesseasily combustible than the body of the carbon b so that a well taperedpoint may be secured, but such carbons have the disadvantage that thesoft outer shell burns away too rapidly, and leaves a long narrowportion consisting mainly of a hard core (Fig. 2) which some- -timeseven breaks off at the holder c Fig. 2.

It has also been proposed to employ a carbon made of a hard core, a moreeasily combustible shell and an outer coating of copper, the latterbeing employed for the purpose of increasing the conductivity of thecarbon and. therefore necessarily being of some substantial thickness.The copper covering employed by me is, however, a mere ilm serving onlyas a means for predetermining or adjusting the rate at which the outershell will burn away.

I find that the desired results are obtained with a negative carbonlconsisting of the combination of the following: 1. A centi'al core ofhard carbon. 2. A114 outer shell The thickness of the hard core andouter shell must of course depend upon the cur rentl that is requiredand I find that the thickness of the copper skin may also be varied withadvantage Within certain limits. The lower of these limits is determinedby the thickness required to withstand the necessary handling andpacking; the upper limit depends upon the thickness which will result inthe outer shell not burning away sufficiently far to allow 'the core toprotrude well beyond it.l Within these limits quite a small variation inthe thickness of the metallic skin makes a suiicient variation in thelength of the carbon tip exposed to cause a considerable change in therate of burning of the carbon. The thinner the copper all other thingsbeing equal the longer is the exposed tip and the faster therefore doesthe carbon burn. This leads to the following important result: I findthat by choosing the right thickness of core, outer shell, and copperskin, it is always possible to obtain a negative carbon that will burnsteadily at substantially the same rate as any positive carbon suitablefor the current and pressure employed.

A negative carbon constructed according to this invention is shown inFig. 3 of the accompanying drawings. There is a central core a of hardcarbon, an outer shell b of carbon which is made to burn away morequickly than the core and a thin incasing lm c of copper, or othermetal.

The copper coating here employed is not used, as such coatings havehitherto been, to reduce the size of the carbon by-increasing itsconductivity.- So long as the carbon burns with such a tip that the coreprotrudes wellv beyond the softer outer shell, its size elsewhere isunimportant. The sole object of the copper film is to form a tightlyadhering incombustible skin over the carbon,which, while protecting themajor porouter shell vand a such as 1s describe tion of it fromburning,shall always melt off and volatilize at such a distance from the tip as`to allow the point to be well tapered, and to insure that none of themelted or volatilized metal shal'l enter the arc. A carbon with ahardcore, a more'combustible protective metallic skin,

above, burns with perfect steadiness during the'whole of its life. Alsothere is so little copper thatmost of it volatilizes as it melts anddoes not drop oit' or splash the lens or mirror as is the case wherelthick 'copper coverings are employed and the copper melts at such adistance along the carbon that'none vof, themelted or volatilizedvmetalfenters the arc. Fig. 3 of the drawings shows ,'anexample'of theshape assumed during burning of a negative carbon made in accordance.with this invention.

Theouter shell and the central hard core need not necessarily -be'inengagement since a film or skin of copper may be'interposed betweentheml i I find that in a comparatively cool lamphouse with a current-fof 85 amperes, a 15 mm. negative carbon having a hard core 4 mm. indiameter, and a skin or coating of copper .02 mm. in thickness burnsperfectly steadily with a. cored positive carbon 25 mm.` in diameterduring the whole of both of their lives, and they both burn atsubstantially the same rate.

Other metals besides copper may be employed to form the incombustiblecoating, though copper has so far been found the most useful.

What I claim and desire to secure by Letters Patent is 1. A negativecarbon for arc lights consisting of the combination of a hard core, asofter or more readily combustible shell and an outer film of copper orsome other metal, the whole being such that the metal, while forming anadherent incombustible skin over the carbon, melts and volatilizes atsuch a distance from the tip that the melted and volatilized metal istoo far from the arc to enter it, and that the b ared carbon burns sothat the core always protrudes well beyond the outer shell and the arcremains steadily on the protruding end without moving substantially asset forth.

2. A negative carbon for arc lights, consisting of the combination of ahard core, a softer or more readily combustible shell and an outer filmof copper or some other metal, the whole being such that the metal,while forming an adherent incombustible lskin over the carbon, melts andvolatilizes at such a distance from the tip that the melted andvolatilized metal is too far from the arc to enter it, and that thebared carbon burns so that the core always protrudes well beyond theouter shell and the arc remains 115 steadily lon the protruding endwithout moving, the parts of saidcarbon being so proportioned that itwill burn away at the same rate as any positive carbon suitable for thecurrent employed substantially as set 120 forth.

3. A negative carbon for 'electric lights comprising a hard core, asofter or more readily combustible shell and a film of incombustiblef'volatilizable metal disposed 125 upon such slfell.

4. A neg tive carbon for electric lights comprising a hard core, asofter or more readily co bustible shell and a film of incombustiblvolatilizable metal disposed 130 upon such shell, the Various parts ofthe my name in the presence of two subscribing structure belng of suchrelatlve proportlon Witnesses. that the same W111 burn away at the sameHERTHA AYRTON. rate of speed as any posltlve carbon sultable 5 for thecurlent employed, substantially as Witnesses:

set forth. G. V. SYMEs,

In Witness whereof I have hereunto signed 'i ALFRED B. CAMPBELL.

